1. Field of the Invention
The present invention relates to a varistor (variable resistor), specifically to a varistor mainly composed of zinc oxide and a method of manufacturing the same.
2. Description of the Related Art
A varistor is a circuit element whose resistance abruptly decreases when the voltage applied to the element exceeds a predetermined level. On the other hand, the resistance is extremely large when the applied voltage is lower than that level. Because of such characteristics, varistors are used for protecting semiconductor elements from a surge voltage, for example.
Resistance elements referred to as zinc oxide varistors which utilize voltage/current non-linearity are mainly composed of zinc oxide and are manufactured using a method wherein zinc oxide is combined with a plurality of additives, pulverized and mixed; the resultant fine particles are calcined to produce a ceramic raw material powder for varistors; and a body molded from the ceramic raw material powder is fired to provide varistor elements. Energy barriers are formed in boundary barrier layers in such a varistor element due to the presence of impurity energy levels formed at boundaries between zinc oxide particles, which results in excellent voltage/current non-linearity.
The voltage of a varistor element at the time when it exhibits the voltage/current non-linearity is referred to as the xe2x80x9cvaristor voltagexe2x80x9d, and the voltage that appears on an element when a current of 1 mA flows through the element is generally used as such a varistor voltage. Such a voltage is normally represented by xe2x80x9cV1mAxe2x80x9d.
The varistor""s function of absorbing surge noises and electrostatic noises are attributable to its excellent voltage/current non-linearity. The surge noise absorbing capability of a varistor is evaluated using a current value at which the varistor element is broken when a sequentially increased surge current is applied to the varistor element. It serves as an important index that indicates the durability of a varistor element.
A varistor mainly composed of zinc oxide has excellent surge resisting capability which increases in proportion to the area of its electrode. However, no compact varistor element capable of resisting surge noises of 3000 A/cm2 or more has been provided.
Meanwhile, there is an accelerated trend toward smaller electronic apparatus, e.g., the spread of mobile communication apparatus, which is accompanied by significant trend toward smaller electronic components and devices of various types. While this has resulted in an increased demand for smaller varistors, a reduction in the size of the varistor leads to a reduction in the effective area of the electrode thereof. For this reason, a need has arisen for varistor elements having improved surge resisting capability per unit area.
It is therefore an object of the present invention to provide a varistor having high surge resisting capability in spite of a reduced size and a method of manufacturing the same.
According to a first aspect of the invention, there is provided a ceramic composition characterized in that it is mainly composed of zinc oxide and contains a composite oxide expressed by the chemical formula Zn2SnO4.
According to a second aspect of the invention, there is provided a ceramic composition characterized in that it contains the composite oxide expressed by a chemical formula Zn2SnO4 in an amount in the range from about 0.01 to 20 mol %.
According to a third aspect of the invention, there is provided a ceramic composition characterized in that it is mainly composed of zinc oxide and contains bismuth oxide, cobalt oxide, manganese oxide and a composite oxide expressed by the chemical formula Zn2SnO4.
According to a fourth aspect of the invention, there is provided a ceramic composition characterized in that it contains about 0.10 to 2 mol % bismuth oxide, about 0.10 to 2 mol % cobalt oxide, about 0.10 to 2 mol % manganese oxide and about 0.01 to 20 mol % Zn2SnO4.
According to a fifth aspect of the invention, there is provided a varistor characterized in that it comprises a layered body formed by a plurality of ceramic layers mainly composed of zinc oxide containing a composite oxide expressed by the chemical formula Zn2SnO4, an inner electrode layer interposed between the ceramic layers of the layered body and an outer electrode formed on the surface of the layered body and electrically connected to the inner electrode layer.
According to a sixth aspect of the invention, there is provided a varistor characterized in that it contains the composite oxide expressed by the chemical formula Zn2SnO4 in an amount from about 0.01 to 20 mol %.
According to a seventh aspect of the invention, there is provided a varistor characterized in that it comprises a layered body formed by a plurality of ceramic layers mainly composed of zinc oxide containing bismuth oxide, cobalt oxide, manganese oxide and a composite oxide expressed by the chemical formula Zn2SnO4, an inner electrode layer interposed between the ceramic layers of the layered body and an outer electrode formed on the surface of the layered body and electrically connected to the inner electrode layer.
According to an eighth aspect of the invention, there is provided a varistor characterized in that it contains about 0.10 to 2 mol % bismuth oxide, about 0.10 to 2 mol % cobalt oxide, about 0.10 to 2 mol % manganese oxide and about 0.01 to 20 mol % Zn2SnO4.
According to a ninth aspect of the invention, there is provided a method of manufacturing a varistor characterized in that it comprises the steps of mixing zinc oxide and tin oxide, thermally processing the mixture thereafter to obtain a composite oxide expressed by the chemical formula Zn2SnO4, and combining the composite oxide with the zinc oxide which is the main component and thermally processing the combination to obtain a raw material for a varistor.
According to a tenth aspect of the invention, there is provided a method of manufacturing a varistor characterized in that the mixing ratio between the zinc oxide and tin oxide is about 2:1 in terms of mol ratio.
According to an eleventh aspect of the invention, there is provided a method of manufacturing a varistor characterized in that the mixture is subjected to a thermal process at 1100xc2x0 C.
According to a twelfth aspect of the invention, there is provided a method of manufacturing a varistor characterized in that the composite oxide expressed by Zn2SnO4 is in an amount in the range from about 0.01 to 20 mol % relative to the zinc oxide which is the main component.
As described above, there is provided according to the present invention varistors and chip type varistors having high surge resisting capability by adding a composite oxide expressed by Zn2SnO4 to the zinc oxide which is the main component.
Further, by adding bismuth oxide, cobalt oxide and manganese oxide in addition to Zn2SnO4, there is provided varistors and chip type varistors which have not only improved surge resisting capability but also has excellent electrical characteristics such as non-linearity coefficient, electrostatic capacity and dielectric loss.
Further, according to the method of manufacture wherein zinc oxide and tin oxide are mixed; the mixture is subjected to a thermal process thereafter to obtain a composite oxide expressed by the chemical formula Zn2SnO4; and the composite oxide is added to the zinc oxide which is the main component and a thermal process is performed to obtain a raw material for a varistor, there is provided a raw material for a varistor which exhibits high surge resisting capability to allow manufacture of a reliable varistor.